linux-kernel - Re: [PATCH v8 6/7] apei/ghes: Use unrcu

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Message-ID: <Y0g+wv1AB1dHhvq+@zn.tnic>
Date:   Thu, 13 Oct 2022 18:37:22 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Justin He <Justin.He@....com>, Len Brown <lenb@...nel.org>,
        James Morse <James.Morse@....com>,
        Tony Luck <tony.luck@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Robert Richter <rric@...nel.org>,
        Robert Moore <robert.moore@...el.com>,
        Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Jan Luebbe <jlu@...gutronix.de>,
        Khuong Dinh <khuong@...amperecomputing.com>,
        Kani Toshi <toshi.kani@....com>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "devel@...ica.org" <devel@...ica.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Shuai Xue <xueshuai@...ux.alibaba.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        nd <nd@....com>, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

On Thu, Oct 13, 2022 at 05:41:06PM +0200, Ard Biesheuvel wrote:
> No it definitely does not imply that. A memory clobber is a codegen
> construct, and the hardware could still complete the writes in a way
> that could result in another observer seeing a mix of old and new
> values that is inconsistent with the ordering of the stores as issued
> by the compiler.

Yes, but look at the code. There's a:

	smp_wmb();

which on x86 is

	#define smp_wmb() barrier()

which is

	#define barrier() __asm__ __volatile__("": : :"memory")

so there wasn't a hardware memory barrier there in the first place.

Unless ARM does something else in those primitives...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette